The electric power system in the United States generates three-phase alternating current (AC) electric power. Each power phase is 120 degrees out of phase, plus or minus, with the other two power phases. The voltage of any phase oscillates sinusoidally between positive voltage and negative voltage. It happens that three-phase electric power generation, transmission, and distribution provides an acceptable compromise between the efficiency, expense, and complexity of power system equipment.
It is more efficient to transmit electric power at high voltage levels than at low voltage levels. Electric power may be generated as three-phase AC power at moderate voltage levels in the 12 thousand volt (kV) to 25 kV range. The voltage level may be stepped up to the 110 kV to 1000 kV range using a transformer for transmission over long transmission lines, hence minimizing transmission line power loss. The transmission line voltage may be stepped down, using a transformer at a substation, to the 12 kV to 35 kV range for local distribution. The local distribution voltage level may be further stepped down through one or more transformer stages to provide 120 volt AC power to the home and office. Special accommodations may be made for manufacturing plant electric power consumers. In some contexts, the electric power system may be abstractly categorized into electric power generation, electric power transmission over extended distances, and electric power distribution to electric power consumers.
The performance of an insulation system in generators, motors, transformers, bushings, and other high-voltage components may change as the insulation deteriorates. This deterioration may lead to a dissipation factor (DF) that is greater than zero. In some contexts, the dissipation factor may be referred to as the insulator power factor. In practical power system components, the dissipation factor is always greater than zero, but by a tolerable fraction of a percent. For example, a dissipation factor value at 20 degrees C. for a new power transformer insulation system may be about 0.002. A dissipation factor value of 0.01 may be grounds for an alert or warning. The external connections to power transformer windings may be provided via bushings. In some embodiments, bushings include ceramic insulators.
Testing of power system transformers may be conducted by connecting a test set to the windings of the power system transformers and exciting the primary winding and the secondary winding with electric signals, both direct current and alternating current. Testing may be conducted on one transformer phase at a time, or may be conducted on multiple transformer phases concurrently. From some points of view, testing electric generators has some similarities to testing transformers. An exciter winding in a generator may be considered to be similar, in some respects, to a transformer winding. The windings of a generator may be considered to be similar, in some respects to a transformer winding. Likewise, from some points of view, testing electric motors may have some similarities to testing transformers and/or testing generators. Transporting the power system transformer, generator, or electric motor to a controlled test laboratory environment may not be economically feasible, and therefore testing typically occurs on site, often outdoors in variable weather conditions. As can readily be appreciated by one skilled in the art, the testing environment associated with high voltage power system transformers, electric generators, and electric motors may be subject to intense electric field fluxes as well as high levels of air borne dust and grit.